Rayya is one of the few followers in Skyrim who will dual wield as long as shields and two-handed weapons are not in her inventory. Like all followers who are capable of dual wielding, Rayya must be in actual combat before she will unsheathe the second weapon but will only unsheathe one weapon while out of combat.

Rayya's two starting scimitars are listed twice in the inventory menu, but each listing is "stacked", showing "Scimitar (2)". When you remove one, a "stack" of two is removed; the same applies to the second "stack".